There is a rising demand in recent years in the welder industry for improvement of arc-starting performance in order to increase the productivity by shortening welding time and to obtain high quality in the welding result. One of the methods of shortening the welding time is to reduce a processing time to end the welding. Also useful is to optimize a shape of the tip of the wire at the end of welding, as one of the methods of improving the arc-starting performance. The both methods pertain to control process at the end of the welding.
FIG. 4 is a schematic diagram showing a general structure of a conventional arc welding apparatus. Primary rectifier element 3 rectifies the power input from mains power source 1. Switching element 4 switches an output of primary rectifier element 3 to produce an output suitable for welding. Main power transformer 2 converts the output of switching element 4 into an output suitable for welding. Secondary rectifier element 6 rectifies the output of main power transformer 2. Reactor 5 smoothes the output of secondary rectifier element 6 into a current form suitable for welding. Welding current detector 8 detects a welding current. Welding voltage detector 9 detects a welding voltage. Short/arc detector 10 determines whether welding condition is in a short mode, in which wire 16 and base metal 15 are in contact with and stay shorted, or in an arcing mode, in which they have opened out of the short mode and arc is being generated, according to a welding voltage detector signal. Welding start dictator 35 inputs a welding start signal and a welding end signal to welding power unit 14 from the outside. Welding-end determining section 34 determines an end point of the welding time based on the input from welding start dictator 35. Integrator section 30 computes an integrated amount of welding current, starting from a time immediately after opening of the shorting at the end of welding portion. Threshold value setting section 31 sets a threshold value for comparison with the integrated amount of welding current. Comparator section 32 compares the integrated amount of welding current with the threshold value. Output controller 36 outputs a signal for controlling welding power output. Driver 33 outputs a signal for driving switching element 4 to control the welding power output.
Welding voltage detector 9 is connected across output terminals of welding power unit 14, and outputs a signal corresponding to a detected voltage. Short/arc detector 10 judges based on the signal from welding voltage detector 9 as to whether the welding power output voltage is at least equal to or less than a predetermined value. Using a result of this judgment, short/arc detector 10 determines whether wire 16 is in a short mode by being in contact and shorted with base metal 15 of the object to be welded, or an arcing mode by being not in contact with base metal 15, and it then outputs a determination signal.
FIG. 5 is a graphic representation illustrating waveforms of wire feeding speed Wf, welding voltage Vw and welding current Aw in the conventional method of arc welding.
Referring now to FIG. 4 and FIG. 5, description is provided of the conventional method of controlling the arc welding. When a welding end signal is input from welding start dictator 35 at time T1, welding-end determining section 34 detects a point of time from which to start controlling the end of welding. The wire feeding speed is then decelerated at a predetermined inclination toward a stop. When a change occurs in the detection from the short mode to the arc mode, short/arc detector 10 determines this point as to be time T2 for a globule on the tip end of wire 16 to move to base metal 15. Taking time T2 as a starting point, integrator section 30 starts integrating output current to obtain an integrated value, and comparator section 32 compares this integrated value with the threshold value set by threshold value setting section 31. When the integrated value reaches the threshold value, the output current of a predetermined value set beforehand by output controller 36 is supplied for a predetermined time period t1 to form a globule at the tip end of wire 16, and the welding power output is terminated (refer to patent literature PTL1 for instance).
It becomes possible by the above method to control a shape of the tip end of the wire to have a uniform size without variations at the end of welding, and limit the influence of slag upon starting the subsequent arc, thereby achieving an excellent arc-starting performance.
According to the conventional method of welding control discussed above, it is possible to control the shape of tip end of the wire to be uniform in size without variations. However, when attempted to control and form a globule of a small size (e.g., about 1.1 to 1.3 times the diameter of the wire) on the tip end of wire 16, there are cases wherein a stick or the globule is absorbed into a molten metal pool due to heaving and the like of the molten metal pool, resulting in a failure to form the globule of desired size.